A hard, nacreous, iridescent body formed by layers of finely crystalline calcium carbonate deposited in concentric layers inside the shell of a living freshwater or marine mollusc to protect the animal from damage to the mantle or to isolate an irritant. Pearls of high quality formed in the wild are gemstones, and rare. Pearls are cultured by injecting a small piece of mantle tissue from a donor mollusc into the shell of a recipient, triggering the formation of a protective pearl sac and then a pearl.